The invention relates to an optical fiber coating composition. In particular, the invention relates to a radiation-curable primary or secondary optical fiber coating composition comprising oligomers endcapped on each end with either a photoinitiator group or an adhesion promoter group, and/or oligomers endcapped on one end with a photoinitiator group and endcapped on the other end with an adhesion promoter group.
Optical fibers made from drawn glass have been used as a reliable transmission medium in telecommunications cables. Glass optical fibers are employed because they have the ability to carry large amounts of information over long distances.
To facilitate these long-distance transmissions, optical fiber waveguides have been coated with plastic compositions of various materials in order to protect the fiber. Optical glass fibers are often coated with two superposed coatings. The coating which contacts the glass is a relatively soft, primary coating that must satisfactorily adhere to the fiber and be soft enough to resist microbending, especially at low service temperatures. The outer, exposed coating is a much harder secondary coating, and it provides resistance to handling forces, while possessing sufficient flexibility to enable the coated fiber to withstand repeated bending without cracking the coating.
Optical fiber coating compositions, whether primary coating compositions, single coating compositions, or secondary coating compositions, typically comprise, prior to being cured, one or more oligomers, photoinitiator(s) or a photoinitiator system, adhesion promoter(s), and reactive diluent(s). Optical fiber coating compositions also typically comprise flow control additive(s), antioxidant(s), and stabilizer(s).
The coating composition is applied to the optical glass fiber in a liquid state, and is thereafter exposed to actinic radiation to effect cure. Specifically, coatings are applied to the fiber in-line during fiber drawing. As the state of fiber drawing technology has allowed for increased draw speeds to effectuate longer optical fibers, however, the need for coating compositions that can cure at faster rates coincident with the faster draw speeds has become more urgent. Thus, as draw speeds have increased, a need has developed for materials that cure at faster rates than is presently available with current technology.
One method of effecting photopolymerization, and thereby curing the coating composition, is to create free radicals. It is known that a photoinitiator(s) or photoinitiator system may be included in a coating composition in order to generate free radicals upon the absorption of light, e.g. ultraviolet light. When the photoinitiator molecule absorbs radiation, electrons are promoted to higher energy levels in the molecule and the molecule becomes unstable, i.e. very reactive. So, for example, a photoinitiator molecule becomes electronically excited upon the absorption of light and undergoes alpha-cleavage to form two free radicals. One or both of these free radicals are capable of initiating polymerization. Other more complicated processes employ photoinitiator systems, which include co-initiators, from which a photoinitiator molecule may abstract a hydrogen (electron) to form a radical pair. Amines are typical co-initiators.
Although the prior art discloses various curable coating compositions with photoinitiators or a photoinitiator system dissolved therein, the effectiveness of the photoinitiators has not always been maximized. For example, problems sometimes exist with initiating the cure. In addition, free residual photoinitiator has a tendency to leach from the cured composition, thereby deteriorating the coating. Still further, photoinitiators may sometimes remain as low molecular weight inclusions in the cured polymer, thereby reducing the maximum physical properties obtainable in the cured system by adversely affecting properties such as hardness, abrasion resistance, etc.
A similar problem exists for the inclusion of one or more adhesion promoters in a coating composition. In the case of primary coating compositions, it is important that the cured coating adhere well to the glass cladding. Otherwise, the cured coating will have a tendency to delaminate from the glass fiber. Although the prior art discloses various curable coating compositions with adhesion promoters dissolved therein, their effectiveness has not always been maximized. Thus, a need still exists for adhesion promoters that provide even stronger adhesion to glass at relatively low concentrations.
There is provided a radiation-curable coating composition comprising oligomers that each have two end groups X1 and X2, wherein the end groups X1 and X2, which may be the same or different, are selected from the group consisting of a photoinitiator group and an adhesion promoter group.
An oligomer according to the present invention having a photoinitiator group as at least one of the end groups X1 and X2 is an oligomeric photoinitiator that generates free radicals upon exposure to a sufficient amount of actinic radiation. An oligomer according to the present invention having an adhesion promoter group as at least one of the end groups X1 and X2 is an oligomeric adhesion promoter that, when present in a radiation-curable coating composition, provides the coating composition with strong adhesion to glass. A particular oligomer of the present radiation-curable coating composition having, for example, a photoinitiating group X1 and an adhesion promoter group X2, i.e. a particular oligomer for which X1 and X2 are not the same, may be referred to as an oligomeric photoinitiator/adhesion promoter.